In eukaryotic cells DNA is packaged with histones, to form chromatin. Approximately 150 base pairs of DNA are wrapped twice around an octamer of histones (two each of histones 2A, 2B, 3 and 4) to form a nucleosome, the basic unit of chromatin. The ordered structure of chromatin needs to be modified in order to allow transcription of the associated genes. Transcriptional regulation is key to differentiation, proliferation and apoptosis, and is, therefore, tightly controlled. Control of the changes in chromatin structure (and hence of transcription) is mediated by covalent modifications to histones, most notably of the N-terminal tails. Covalent modifications (for example methylation, acetylation, phosphorylation and ubiquitination) of the side chains of amino acids are enzymatically mediated (A review of the covalent modifications of histones and their role in transcriptional regulation can be found in Berger S L 2001 Oncogene 20, 3007-3013; See Grunstein, M 1997 Nature 389, 349-352; Wolffe A P 1996 Science 272, 371-372; and Wade P A et al 1997 Trends Biochem Sci 22, 128-132 for reviews of histone acetylation and transcription).
Acetylation of histones is associated with areas of chromatin that are transcriptionally active, whereas nucleosomes with low acetylation levels are, typically, transcriptionally silent. The acetylation status of histones is controlled by two enzyme classes of opposing activities; histone acetyltransferases (HATs) and histone deacetylases (HDACs). In transformed cells it is believed that inappropriate expression of HDACs results in silencing of tumour suppressor genes (For a review of the potential roles of HDACs in tumorigenesis see Gray S G and Teh B T 2001 Curr Mol Med 1, 401-429). Inhibitors of HDAC enzymes have been described in the literature and shown to induce transcriptional reactivation of certain genes resulting in the inhibition of cancer cell proliferation, induction of apoptosis and inhibition of tumour growth in animals (For review see Kelly, W K et al 2002 Expert Opin Investig Drugs 11, 1695-1713). Such findings suggest that HDAC inhibitors have therapeutic potential in the treatment of proliferative diseases such as cancer (Kramer, O H et al 2001 Trends Endocrinol 12, 294-300, Vigushin D M and Coombes R C 2002 Anticancer Drugs 13, 1-13).
In addition, others have proposed that aberrant HDAC activity or histone acetylation is implicated in the following diseases and disorders; polyglutamine disease, for example Huntingdon disease (Hughes R E 2002 Curr Biol 12, R141-R143; McCampbell A et al 2001 Proc Soc Natl Acad Sci 98, 15179-15184; Hockly E et al 2003 Proc Soc Natl Acad Sci 100, 2041-2046), other neurodegenerative diseases, for example Alzheimer disease (Hempen B and Brion J P 1996, J Neuropathol Exp Neurol 55, 964-972), autoimmune disease and organ transplant rejection (Skov S et al 2003 Blood 101, 14 30-1438; Mishra N et al 2003 J Clin Invest 111, 539-552), diabetes (Mosley A L and Ozcan S 2003 J Biol Chem 278, 19660-19666) and diabetic complications, infection (including protozoal infection (Darkin-Rattray, S J et al 1996 Proc Soc Natl Acad Sci 93, 13143-13147)) and haematological disorders including thalassemia (Witt O et al 2003 Blood 101, 2001-2007). The observations contained in these manuscripts suggest that HDAC inhibition should have therapeutic benefit in these, and other related, diseases
Many types of HDAC inhibitor compounds have been suggested, and several such compounds are currently being evaluated clinically, for the treatment of cancers. For example, the following patent publications disclose such compounds:
U.S. Pat. No. 5,369,108U.S. Pat. No.WO 01/70675and WO 01/181714,254,220WO 01/38322WO 02/30879WO 02/26703WO 02/069947WO 02/26696WO 03/082288WO 02/22577WO 03/075929WO 03/076395WO 03/076400WO 03/076401WO 03/076421WO 03/076430WO 03/076422WO 03/082288WO 03/087057WO 03/092686WO 03/066579WO 03/011851WO 04/013130WO 04/110989WO 04/092115WO 04/0224991WO 05/014588WO 05/018578WO 05/019174WO 05/004861WO 05/007091WO 05/030704WO 05/013958WO 05/028447WO 05/026907
Many of the HDAC inhibitors known in the art have a structural template, which may be represented as in formula (A):

wherein ring A is a carbocyclic or heterocyclic ring system with optional substituents R, and [Linker] is a linker radical of various types. The hydroxamate group functions as a metal binding group, interacting with the metal ion at the active site of the HDAC enzyme, which lies at the base of a pocket in the folded enzyme structure. The ring or ring system A lies within or at the entrance to the pocket containing the metal ion, with the -{Linker]- radical extending deeper into that pocket linking A to the metal binding hydroxamic acid group. In the art, and occasionally herein, the ring or ring system A is sometimes informally referred to as the “head group” of the inhibitor.
The use of prodrugs to enhance the delivery to target organs and tissues, or to overcome poor pharmacokinetic properties of the parent drug, is a well known medicinal chemistry approach. Administration of ester prodrugs, for example, which are hydrolysed by serum carboxylesterases in vivo to the active parent acids, can result in higher serum levels of the parent acid than administration of the acid itself.